Direct-coupled amplifier



Jan. 8, 1957 A. W. RUSSELL DIRECT-COUPLED AMPLIFIER Filed Oct. 15, 1954 IN VEN TOR.

ALFRED W. RUSSELL ATTORNEYS United States Patent DIRECT-COUPLED AMPLIFIER Alfred W. Russell, Waldwick, N. 1., assignor to Allen B. Du Mont Laboratories, Inc., Clifton, N. J., a corporation of Delaware Application October 15, 1954, Serial No. 462,509

2 Claims. Cl. 179-171 This invention relates to direct-coupled amplifiers and in particular to a method of compensating for variations of voltage occurring in the associated power supply circuits.

One of the applications of a direct-coupled amplifier is in oscillography, where the output of the amplifier is applied to the deflection plates of a cathode-ray tube to produce a displacement of the spot proportional to the signal voltage applied to the input circuit of the amplifier. In order to maintain a truly linear relationship between output and input signals, it is standard practice to provide a negative feedback loop for signal degeneration and this has the effect of stabilizing the amplifier against power supply variations. The accelerating potentials applied to the cathode-ray tube are normally derived from power supplies whose output voltage will vary at the same time and in the same way as those applied to the amplifier. Since changing the accelerating potentials applied to the cathode-ray tube varies the deflection sensitivity or stiffness of the beam, this changes the constant of proportionality between spot displacement and input signal voltage.

It is obviously possible to maintain a constant deflection sensitivity by regulating the cathode-ray tube supply voltages, but this of course adds to the bulk and cost of the instrument. This invention discloses a novel solution in which the gain of the amplifier is made to vary with the supply voltage in such a way as to compensate for the changing deflection sensitivity of the cathode-ray tube.

In the accompanying drawings,

Figure 1 is a cut-away diagrammatic view of a cathoderay tube, and T Figure 2 is a schematic diagram of one embodiment of the circuit of the instant invention.

For ease of explanation, the following terms are hereby defined.

Upvolting means raising the potential, but not necessarily to a positive value.

Downvolting means lowering the potential, but not necessarily to a negative value.

Referring to Figure 1, there is shown a cross-sectional view of a cathode-ray tube having a glass envelope 10 and an electron gun structure 11 which produces a stream of electrons 12. These impinge upon a fluorescent screen 13 deposited on the inner surface of the faceplate of tube 10. In order to develop a pattern on the fluorescent screen, the electron beam 12 is deflected, the figure illustrating one set of deflection plates 14. Generally, additional sets are required, or magnetic coils may be used.

Shown in Figure 2 is the circuit of the invention. Broadly speaking, it operates so that an increase of accelerating voltage causes the amplifier to operate at a state of higher gain. In this manner, a stronger deflection force is produced to overcome the stiffer electron beam.

Reference character 20 indicates a source of deflection signals. Assuming for the moment a constant potential E at terminal 27, a constant potential V at terminal 23, and a constant potential at point A, the input signal IS 2,777,018 Fatented Jan. 8, 1957 ICC amplified by tubes 48 and 52 and then appears across load resistors 21 and 22, and also at control grids 31 and 32. The signals are further amplified by electron tubes 23 and 24 and appear at output terminals 25 and 26 from whence they are applied to deflection plates similar to those shown at 14 of Figure 1.

Electron tubes 23 and 24 have a common cathode resistor in the form of electron tube 33. Screen grid 34 of this tube is connected to the terminal 27 where the voltage E is also applied. Control grid 36 connects to the movable contact of a potentiometer 37 which is part of a voltage divider comprising resistors 38, 39, 40 and 41. A source of potential 55 returns the voltage divider to ground in such a manner that the bias voltage applied to control grid 36 is sufiiciently negative to meet the requirements of tube 33.

Resistor 45 which is connected between the anode 46 of tube 23 and the control grid 47 of tube 48 is a degenerative connection. Similarly, resistor 58 is the degenerative connection between anode49 of tube 24 and control grid 51 of tube 52. These resistors form the amplifier stabilizing degenerative feedback loop previously mentioned, and in conjunction with resistors 53 and 54 determine the amount of feedback. Screen resistor 56 is also part of the stabilizing circuit. Since these elements are not related to the instant invention, their action will not be explained in detail.

It will be noted that the unregulated potential V at terminal 28 is applied to the amplifier (tubes 23 and 24), which is stabilized by the elements described above, and the effects caused by any fluctuations of V are thereby minimized, or eliminated. The same potential V is also utilized as the accelerating voltage for the cathode-ray tube, not shown, and being unregulated varies the stiflfness of the electron beam as previously explained.

If the potential V at terminal 28 increases for any reason, for example fluctuation of the input line voltage, control grid 36 is upvolted, due to the action of the voltage divider. The amplitude of upvolting depends upon the elements comprising .the voltage divider, and upon the setting of potentiometer 37. The upvolting of control grid 36 permits an increase in conductivity of tube 33 which decreases the voltage drop across the tube. This effectively downvolts anode 42 (and the cathodes 43 and 44 which are connected thereto) relative to the grids and cathode of tube 33.

As is well known in the art, the gain of an amplifier varies directly with the transconductance, which in turn varies inversely with the grid bias.

1 GaG aE Thus, the downvolting of cathodes 43 and 44 decreases the bias on control grids 31 and 32, and in accordance with the above equation increases the gain so that the signal is more strongly amplified. Thus, an increase in voltage V automatically produces a stronger deflection signal to overcome the stitfening of the electron beam. Conversely, a decrease of the voltage V will weaken the amplified signal to compensate for a softer electron beam.. This circuit will compensate for variation as high as il0% of the voltage V. By proper selection of circuit constants these effects may be intensified or even reversed.

The above description sets forth one specific example of the invention herein disclosed, but those skilled in the art will readily appreciate that the subject matter thereof can be incorporated in various forms. I prefer, therefore, to be limited only as required by the appended claims.

What is claimed is:

1. A direct coupled amplifier whose gain is compensated for fluctuations in its unregulated supply, comprising: a pair of electron tubes connected in push-pull relationship (each said tube having an anode, a cathode, and a control grid); a source of balanced out-of-phase input signals applied to respective said grids; an unregulated direct potential source connected to said anodes; av direct connection between said cathodes; a variable common cathode resistance consisting of a single pentode (having an anode connected to said cathodes, a cathode connected to ground, and a control grid); means to control the gain of said amplifier in accordance with fluctuations of said unregulated source, said means comprising a bias source and a potentiometer connected together as a voltage divider across said unregulated source, and means to apply fluctuations of said unregulated source to said control grid of said pentode, said means comprising a connection between the slider of said potentiometer and said control grid of said pentode whereby the resistance of said pentode and thus the gain of said amplifier is varied.

2. A direct coupled amplifier whose gain is compensated for fluctuations in its unregulated supply, comprising: a pair of electron tubes connected in push-pull relationship (each said tube having an anode, a cathode, and a control grid); a source of balanced out-of-phase input signals applied to respective said grids; an unregulated direct potential source connected to said anodes; a direct connection between said cathodes; a variable common cathode resistance consisting of a single pentode having an anode connected to said cathodes, a cathode connected to ground, and a control grid; and means to apply fluctuations of said unregulated source to said control grid of said pentode whereby the resistance of said pentode and thus the gain of said amplifier is varied.

References Cited in the file of this patent UNITED STATES PATENTS 2,158,248 Numans May 16, 1939 2,428,039 Royden Sept. 30, 1947 2,544,340 Maxwell Mar. 6, 1951 2,547,538 Rieke Apr. 3, 1951 2,562,476 Rado July 31, 1951 2,668,272 Groth Feb. 2, 1954 2,721,907 Jacobs Oct. 25, 1955 FOREIGN PATENTS 540,834 Great Britain Oct. 31, 1941 

